Maximum fuse size for wire from battery to distribution center?

Jennifer Rose Towing

Established Member
My main question, what is the maximum safe fuse size for wire protection? I have a Buss 150 amp circuit breaker that I would like to use. It is way more then what I currently need but future expansion would make it just rite. It has a manual disconnect I would use that an inline fuse does not offer.

I am in the process of setting up a vehicle. The main power wire from the battery to the distribution/fuse block is an 8 gauge and running about 7 feet. The majority of the everyday equipment will be less then 20 amps but I am putting in an Anderson Power Pole connection for plug n play power for some devices. One being a power inverter that can draw 50 amps. I will likely add a 100 amp external service connection in the future for light vehicle jump starts that would use the same circuit breaker.

I would prefer not to have mutiple circuit breakers/fuses at the battery but questioning if a 150 amp circuit breaker is too big for an 8ga wire? The distribution fuse block is mounted inside the cab and stepped down from there to the devices.
 

Jarred J.

Archive Expert
Silver Supporter
Most jumper cables I've seen that mount to batteries go directly to them... You are talking about the one that has power pole connectors that are placed in the grill with a short lead, and then the rest are about 20 feet long?
 
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Jennifer Rose Towing

Jennifer Rose Towing

Established Member
Yes, but that would be a secondary leg when added. Im fine with the 150a on that line, wondering what the max of the other line could be.
 
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Jennifer Rose Towing

Jennifer Rose Towing

Established Member
Are there giudes somewhere for max fuse to wire ratio? Charts are all over for max amps the wire should run but those vary a lot too. I just have not seen max fuse size. I presume they are close in relation but the fuse size should be bigger.
 

Sparky_911

Member
Gold Supporter
As a safety measure you shouldn't exceed a percentage of rating for wire. As MtnMan said, 8ga 50amps. That is a max rating. Same goes for 10ga/30amp 12ga/20amp, etc. Ok for short bursts (think compressor start up) but run those all the time and you risk fire. So as a safety run a fuse below rated at max amps for the wire or you risk melting the wire before blowing the fuse. As an example I run 15amp fuses on 12ga wire.

You will need to run fused line for each device near 50 amps. As for power poles, mine are unfused 4ga 18inch length from batt to connector and stay underhood. I have 20ft of line that has pole on other end to connect jumper clamps, scene lighting, or viair 2gal compressor one at a time.
 

GE

Registered Member
A 150A breaker should not be used on an 8ga cable. If the maximum rating of the cable is 50A it should have a 50A fuse or breaker on it maximum.

Just like mains voltage structure wiring you should never exceed 80% of the wire's rated capacity. Your breaker panel has 15A breakers for 14ga wire but code says you don't run more than 12A worth of devices off of it. On an 8ga cable your maximum draw should be 40A. If that's not sufficient you need to run a larger cable or multiples of them.

People seem to forget what the fuse is intended to do. It's not protecting your devices from overload (although technically it does that too) it is protecting the WIRE. Any unfused length of wire which can short circuit without protection is a major fire risk. Your fuse rating should match the wire's capacity so that if it suddenly shorts against the frame and starts drawing 100A from the battery (I have seen this in person, looks and sounds exactly like an arc welder) the fuse blows before the wire burns your vehicle up. Fuse as close to the battery as possible first, then branch out from there and protect everything sizing to the WIRE, not the load. Heavier load? Larger cable. Wrong size fuse/breaker? Get a smaller one.

Note that you can always fuse DOWN in rating provided the load is within limits. When I had an 8ga cable off my battery I fused it at 40A for instance. Your main fuse shouldn't be more than 6 inches from the positive battery terminal.
 

Jarred J.

Archive Expert
Silver Supporter
6 inches? Every post on this form has been stated as "within 18 inches" just curious as to where you got the information for your number... You'd be hard pressed to get to a frame wall at 6 inches to mount a circuit breaker as most batteries are at least a foot wide or so.... An not always mounted next to it...
 
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Jennifer Rose Towing

Jennifer Rose Towing

Established Member
Your fuse rating should match the wire's capacity so that if it suddenly shorts against the frame and starts drawing 100A from the battery (I have seen this in person, looks and sounds exactly like an arc welder) the fuse blows before the wire burns your vehicle up.
That is basically what I was curious about. My wire is rated for my maximum load. If I were in an accident or the wire jacket became frayed, the full power of the battery could free flow. With the breaker being a fast blow I was thinking the wire could handle the short surge in amps to trip the breaker. Guess I will bite the bullet and buy another breaker.
 

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